1. Field of the Invention
The present invention generally relates to the testing of manufactured parts for defects. In particular, the present invention relates to laser ultrasound testing methods for testing composite materials and parts made of those materials wherein a series of narrow laser pulses is used to enhance or optimize a resulting ultrasonic signal in the tested material.
2. Description of Prior Art
Laser ultrasound testing methods are non-invasive, generally non-destructive, techniques used to measure various features of materials or parts. These features may include layer thickness, cracks, delamination, voids, disbonds, foreign inclusions, fiber fractions, and fiber orientation. The features may influence a given material""s qualities and performance in given applications. Each application places unique demands on the material""s qualities including the need for differing strength, flexibility, thermal properties, cost, or ultraviolet radiation resistance.
Many techniques are available such as visual, radiographic, thermographic, acoustic-emission, optical, vibrational and ultrasonic methods. Visual techniques examine the surface of an object and infer internal features. Radiographic techniques such as X-ray and gamma-ray techniques detect variations in absorption to determine features. Thermographic techniques observe surface temperatures during heating to determine variations in the materials. Acoustic methods utilize acoustic noise caused by microscopic failures in the material. Optical techniques seek variations in deformations upon stress. Vibrational inspection uses local pulse excitation to exploit the sonic properties of the component. Ultrasonic methods include laser induced and transducer induced techniques and detect features of objects or materials by exploiting differences in acoustic impedances between features.
Laser ultrasound techniques use a laser pulse which when directed at an object causes thermal expansion in a small region. The thermal expansion causes ultrasonic waves which are then measured by a detector and converted into information about the features of the object. The laser pulse may be generated by several lasers. Various types of lasers used include carbon dioxide lasers and a Nd:YAG lasers. The object that is to be tested may be composed of metal, composite materials, ceramic material, or any other type of material. The detector may be a transducer on the surface of the object, a laser interferometer directed at the object, or a gas-coupled laser acoustic detector.
The measured ultrasonic signals are analyzed to reconstruct physical attributes of the object that have a position or location with the object and a size. A combination of the optical penetration depth in the material given the generation-laser optical wavelength and of the temporal profile or pulse width of the generation-laser pulse dictate the frequency content of the laser-generated ultrasonic waves.
One problem associated with many typical ultrasound detectors is a poor signal-to-noise ratio (SNR) in the resulting signals. SNR is proportional to the amplitude of the sonic wave and inversely proportional to the square root of the bandwidth of the sonic wave. Many typical generation techniques produce low amplitude and/or broad bandwidth ultrasonic signals. Either of these conditions lead to an increased SNR and limits the quality of data acquired through such ultrasonic testing. In combination, these effects drastically reduce the ability to detect features in the tested object. The poor SNR in typical systems leads to poor resolution in the resulting analysis. Smaller features like fractures may be difficult to detect with poor resolution. With the lower SNR of the typical systems, these smaller size features may go unnoticed.
When testing fails to detect these smaller features, these features may ultimately yield many problems, such as poor material performance or catastrophic failure of the part. When testing parts used in the airline industry, failure to detect flaws may lead to safety problems and costly accidents.
As such, many typical laser ultrasonic testing devices suffer from low quality measurements caused by low amplitude and high frequency bandwidth. Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.
Aspects of the invention are found in a system for testing physical attributes of a manufactured object. The system has a coherent electromagnetic energy generator coupled to a pulse generator. In combination, these produce a plurality of coherent electromagnetic energy pulses. The plurality of pulses generally have pulse widths of less than 20% the time separation between successive pulses.
The pulses are directed at the manufactured object and impart energy to the manufactured object. The imparted energy then initiates a sonic energy signal in the manufactured object. A detection system detects this sonic energy signal emitted by the manufactured object.
Still, other aspects of the invention include the systems described above where a controlling circuitry may define and/or control the pulse widths of the coherent electromagnetic energy pulses and the time separation between successive pulses. Further, the controlling circuitry may use the physical attribute of the manufactured object or the features of the coherent electromagnetic energy pulses to determine proper or optimal pulse widths and time separation between pulses.
The detection system of the systems described above may be various in nature. They can be transducers, a laser interferometer, an electromagnetic transducer (EMAT), or a gas-coupled laser acoustic detector, to name but a few.
Another aspect of the invention is a method for testing physical attributes of a manufactured object. The method comprises generating a plurality of coherent electromagnetic energy pulses. These pulses have pulse widths less than 20% the time separation between successive pulses. The method calls for directing the pulses at a manufactured object where the pulses impart energy on or in the manufactured objects. This energy initiates a sonic energy signal and the sonic energy signal is detected.
Further aspects of the invention add to the above methods a step where the pulse widths of the coherent electromagnetic energy pulses and the time separation between pulses may be defined and/or controlled. Further, the pulse widths of the coherent electromagnetic energy pulses and the time separation between successive pulses may be defined by the physical attribute of the manufactured object or the features of the coherent electromagnetic energy pulses. The pulses may be defined to generate a specific sonic response in the object.
An exemplary embodiment of the system for testing a physical attribute of a manufactured object may be found in a laser ultrasound testing system. This laser ultrasound testing system has a laser generator, a pulse generator attached to the laser generator, and a sonic energy detection system. The pulse generator produces a plurality of narrow width pulses which are directed at the manufactured object.
The pulses impart energy on or in the manufactured object. This energy initiates a sonic energy signal which is detected by the detection system.
In this case, a plurality of narrow width pulses can be defined as a plurality of laser pulses separated from one another by a time separation between a first pulse and a successive pulse. The temporal profile of the successive pulse is less than 20% the time separation between pulses. The plurality of narrow width pulses may be further described as Dirac-like pulses.
As such, a system for testing manufactured objects using laser ultrasound is described. Other aspects, advantages, and novel features of the present invention will become apparent from the detailed description of the invention when considered in conjunction with the accompanying drawings.